Why You Should Reconsider Planting Peas And Onions Together In Your Garden

A healthy backyard garden should be a biodiverse environment filled with plants that complement each other. That means if one plant is adding a certain nutrient to the soil or repelling a certain bacteria or insect, the plants around it should be able to benefit. While peas and onions go well together in a dish, they should never be grown next to each other in your garden because both plants can be harmed. Onions, as members of the allium family, can get in the way of nitrogen-fixing, a process in which pea plants help convert nitrogen to nitrates and ammonium (the ingredients for chlorophyll and protein production in plants). Without proper nitrogen-fixing, it makes it harder for pea plants to feed themselves. 

To truly get to the root of the problem between onions and peas, you have to go to the roots of the pea plants themselves. While pea plants don't technically "fix" nitrogen, their roots provide a habitat for enzymes found in bacteria, called urease, that convert nitrogen into consumable compounds. As onions grow, they produce a gaseous chemical called allicin, which harms the nitrogen-fixing bacteria that live among the pea roots. This relationship leaves pea plants with a deficiency in the nitrate and ammonium they need to grow. Wilted, yellowing leaves are a telltale sign that your garden is lacking nitrogen.

This detrimental relationship can also harm onions in some cases. Pea plants may produce so much nitrate and ammonium that they could hurt onions. Excessive levels of nitrogen in the soil can inhibit the growth of onions and cause them to not store well after harvest. Furthermore, since pea plants often grow upward by attaching themselves to other plants or posts connected by wire, it's easy for them to block the sunlight of other plants, especially onions. For onions to grow mature bulbs, they need about 12 hours of sunlight a day. Onions grow low to the ground and also have shallow root systems, so they need space to get enough water. Peas on a trellis may block onions from the resources they need, thereby negatively affecting their growth.

Once you have your peas and onions separated, you'll want to surround them with new plants that complement each respective organism, and there are several to choose from. Carrots are a great plant to grow next to onions because their root systems draw from nutrients at different depths, which prevents the species from competing. Carrots also produce aromas that repel pests from your garden. Basil is a great herb to grow next to peas because it wards off thrips, a parasite that embeds its larvae in peas. If you're gardening in a small yard, shade-loving spinach grows across the ground while peas grow upward, so the two plants won't compete for space or resources.